Silicon carbide is one of the most suitable materials for use under severe high temperature condition for example, gas turbine parts or high temperature heat exchangers due to its excellent chemical and physical properties.
Hithertofore, the pressure sintering and reaction sintering processes are widely known for sintering of silicon carbide. However, there have been some disadvantages in these processes, i.e., in the former it is difficult to obtain complicated configuration of the sintered body and of its low productivity, and in the latter it is difficult to obtain the sintered products having high strength and to use under high temperature due to its high content of free silicon.
Meanwhile, silicon carbide is one of variety of materials to be sintered with difficulty, and it has been observed difficult to apply the pressureless sintering generally adopted to manufacture of oxide ceramics wherein a green compact shaped under the normal temperature is sintered without pressure. Recently new pressureless sintering process have been reported wherein a mixed powder comprising silicon carbide, boron containing additive and carbonaceous additive is compacted and sintered under an inert atmosphere. For example, in accordance with U.S. Pat. No. 4,004,934, it is disclosed that silicon carbide is admixed with a boron containing additive (0.3-3.0% by weight of boron) and a carbon containing additive (0.1-1.0% by weight of carbon) is compacted and then sintered without pressure under an inert atmosphere and temperature of 1900.degree.-2100.degree. C. to give a sintered product having at least 85% of theoretical density.
As mentioned above, in the pressureless sintering process of silicon carbide the raw material for making sintered silicon carbide compacts (hereinafter referred to as sintering material) includes a boron containing additives and a carbonaceous additives as main sintering additives, and further includes in some cases, for example U.S. Pat. No. 4,172,109, beryllium etc. The reason for addition of the boron containing additive is to form an adhesive layer around the surface of the silicon carbide particles due to the coexistence of boron and to promote adhesion between the particles and to advance the uniform shrinkage, while the reason for addition of the carbonaceous additive is to remove silica membrane around the silicon carbide particle formed by reduction which hinders the self-sintering and promote the sintering process and at the same time to control the growth of crystalline grain. The carbonaceous material includes various organic substances or inorganic carbon fines. Therefore uniform dispersion of the above-mentioned additives in the sintering material is effective to obtain high quality products.
Meanwhile sintered silicon carbide compacts having high strength and reliability are desired due to the characteristics of silicon carbide. However, known compacts of high strength have been prepared by use of expensive beta-silicon carbide produced by thermal decomposition of organic silicon compound or silicon carbide produced by specific processes disclosed in for example Japanese Patent Disclosure No. 54-67599. It has been observed extremely difficult to produce high quality of sintered products with silica and carbon with conventional manner.
Inventors of the present application have studied the reason of inferior strength of sintered silicon carbide compacts resulting from the conventional process using silica and carbon, and found that inclusion of relatively coarse foreign matters cause pores and faults in sintered compacts resulting in the above-mentioned inferior strength. Thus it is most important that such inclusion should be avoided as possible.
Conventional methods for obtaining the sintering material without inclusion of such foreign matters which deteriorate the strength of sintered silicon carbide compacts are: (1) the production of the sintering material is performed in an isolated chamber without coarse foreign matters or dusts, and (2) foreign matters and dusts are removed by screen classification or sieve separation. However, the method (1) may be practised in a laboratory scale, but in an industrial scale bulk equipments are required and further a complete prevention of the inclusion is almost impossible. The method (2) is classified into wet and dry filtering processes. The dry filtering process is eventually not practical because of aggregating property of fine powders such as the sintering material which makes dispersion of the powders infeasible. On the other hand, in wet filtering process as disclosed in U.S. Pat. No. 4,004,934, the sintering material is mixed in a dispersing medium solution and passed through a sieve. Unfortunately the sieve shown in the above U.S. patent is of over 200 mesh (74 .mu.m) and insufficient to remove foreign matter larger than 65 .mu.m. Thus it can not be expected to appreciably promote the strength by use of sintering material obtained by filtering process.
One of the inventors has proposed an invention disclosed in Japanese Patent Disclosure No. 57-145076 wherein silicon carbide fine powders preferably admixed with sintering additive or shaping additively is shaped into a green compact and then pressureless sintered at least said silicon carbide fine being dispersed in an organic medium to form a suspensoid. The suspensoid is passed through a sieve of less than 44 .mu.m under the influence of ultrasonic vibration, and then the powder is separated from the medium.
In the above-mentioned process it is not adequate to pass the powder through the sieve after admixture of silicon carbide fines, sintering additive and shaping additive because uniform dispersion of silicon carbide fines, sintering additive and shaping additive having respectively different properties such as specific weight and aggregating property for a long period of time resulting in segregation of the additive and varied proportion of silicon carbide fine, sintering additive and shaping additive giving adverse effects on the property of sintered products.
The present invention provides an improvement of the above-mentioned procedure wherein silicon carbide fine, sintering additive, or shaping additive added as required are placed in a dispersing medium solution to form a suspensoid, said suspensoid is treated through a sieve under pressure differential to remove relatively coarse foreign matters effectively.
As object of the present invention is to provide a feasible procedure for manufacturing silicon carbide sintered compact having high strength and reliability.
According to the present invention silicon carbide fine, sintering additive and shaping additive added as required are placed in a dispersing medium solution to form a suspensoid which is treated with a sieve, then the separated particles are formed into a green shape and pressureless sintered, said solid content of silicon carbide fine and sintering additive in the suspensoid being limited within 10-50% by volume, dispersing medium being added to said suspensoid, pressure differential being given to overcome the pressure loss generated between before and after a sieve of less than 65 .mu.m with or without vibration of the suspensoid to remove relatively coarse foreign matters which cause faults of pores and inclusion thereof to achieve the above-mentioned object.